This invention relates to a method and apparatus for testing TFT-LCD's (thin-film transistor liquid crystal displays) used as active-matrix color LCD's.
Recently active-matrix LCD's are extensively used as color displays. The active-matrix LCD writes necessary signals into the liquid crystal only for a given short period of time, holding the written signals during other times by keeping open the gate of the input circuit to liquid crystal. The feature of the active-matrix LCD is that the liquid crystal serves as a dynamic memory.
To ensure that the gate is kept open only for the length of time required for the writing of signals and closed during other times, TFT's (thin-film transistors) or FET's (field-effect transistors) are commonly used.
Recently TFT-LCD's are finding increasing use as active-matrix liquid crystal color displays.
As schematically shown in FIG. 1, a TFT-LCD has many pixels, with a correspondingly large number of TFT's 4 gate lines 3 and data lines 5 allocated to each pixel.
However, there is no sure guarantee that the TFT's 4 always function normally, they are properly connected to the data lines 5 and/or gate lines 3, and the data and gate lines are kept in proper relationship.
Accordingly, pre-assembly test of their functions and arrangement are indispensable to LCD's.
Nevertheless, no efficient testing method has been established for TFT-LCD's. Such being the case, most TFT-LCD's have conventionally been assembled either without such preliminary test or, at best, after checking the relationship of each TFT with its peripheral lines.
One of the methods so far proposed discharges the electricity stored by applying voltage through a data line to an LCD cell capacitor (or an auxiliary capacitor connected in parallel thereto) by controlling the on-off action of a TFT and checks the normality of the individual connections therein by measuring the amount of electric charge induced by the discharging.
This method necessitates an extra integrator (which is, to be specific, an integrating capacitor) and other associated apparatus for the measurement of the charge. Provision of such extra integrator and associated apparatus, however, is troublesome and uneconomical.
The object of this invention is to provide a simple and efficient method and apparatus for checking the normality of the individual elements of TFT-LCD's mentioned before without employing an integrating capacitor and other associated apparatus.